1. Technical Field to Which the Invention Pertains
The present invention relates to a semiconductor device using a compound semiconductor and a method for fabricating the same and, more particularly, to a semiconductor device using a group III nitride semiconductor and a method for fabricating the same.
2. Prior Art
Because of its advantages including a wide band gap, a resultant high breakdown voltage, a high electron saturation velocity, a high mobility, and a high electron concentration in a heterojunction, the research and development of a group III nitride semiconductor have been promoted with a view to applying it to a short-wavelength light emitting element, a high-output RF element, an RF low-noise amplifier element, or the like. To enhance the characteristics of such a group III nitride semiconductor element, it is required to minimize parasitic resistance components such as a contact resistance and a channel resistance in the semiconductor element. In the case of transporting electrons as a current, it is necessary to form an ohmic contact in a region where electrons are conducted (hereinafter generally referred to as an electron channel) from the outside thereof.
FIGS. 11A and 11B are cross-sectional views illustrating a method for forming an ohmic contact in an electron channel in a conventional semiconductor device. FIG. 12 is a cross-sectional view showing an example of the conventional semiconductor device formed with the ohmic contact. A description will be given to a normal method for forming an ohmic contact with reference to these drawings.
First, as shown in FIG. 11A, a gallium nitride (GaN) semiconductor layer 162 as an active layer is formed on a sapphire substrate 161 as a mother substrate. Then, a multilayer metal thin film 163 including Ti as a lowermost layer and Al, Ni, Au, and the like thereabove is formed by a lift-off process on the GaN semiconductor layer (group III nitride semiconductor layer) 162.
Next, as shown in FIG. 11B, annealing is performed at a high temperature (not less than 500° C. and not more than 900° C.), thereby causing a reaction between the Ti layer and N in the GaN semiconductor layer 162 and forming a nitrogen void, while forming a region 164 with an increased metallic property in the vicinity of the upper surface of the GaN semiconductor layer 162. At the same time, Ga and Al also react with N in GaN, similarly to Ti. This lowers a contact resistance between the GaN semiconductor layer 162 and the multilayer metal thin film 165.
For a lower contact resistance, there is also used a method in which the ohmic contact formation region of a semiconductor layer to be brought into contact with a metal film is doped with an n-type impurity at a highest possible concentration prior to the formation of a contact electrode.
In an example shown in FIG. 12, on the other hand, a group III nitride semiconductor layer 172 serving as a channel layer and a barrier layer 173 made of a group III nitride semiconductor larger in band gap than the semiconductor forming the channel layer are provided on a SiC substrate 171 in an ascending order and a gate electrode 177 is provided on the barrier layer 173. In addition, a group III nitride semiconductor layer 176 containing an impurity at a high concentration is formed on the barrier layer 173 and a source electrode 174 and a drain electrode 175 are provided on the group III nitride semiconductor layer 176. To reduce a parasitic resistance in a field effect transistor having such a structure, a method has been used which extensively forms the nitride semiconductor layer 176 heavily doped with an impurity to a position close to the gate electrode or forms a recessed structure. By extending the nitride semiconductor layer 176 which is lower in resistance than the group III nitride semiconductor layer 172 serving as the channel layer to the position close to the gate electrode 177, the resistance of a current flowing between the source and the drain has been lowered. Moreover, reductions in respective resistances between the source and drain and the barrier layer 173 have also been intended.